zebra: clear evpn dup-addr return error-msg when there is no vni

zebra: clear evpn dup-addr return error-msg when there is no vni

clear evpn dup-addr cli returns error-msg for below conditions,

 - If evpn is not enabled &
 - If there is no VNI exists.

supported command:

```
clear evpn dup-addr vni <vni-id>
```

Ticket: #3495573

Testing:

bharat# clear evpn dup-addr vni all
Error type: validation
Error description: % EVPN not enabled

bharat# clear evpn dup-addr vni 20
Error type: validation
Error description: % VNI 20 does not exist

Signed-off-by: Sindhu Parvathi Gopinathan's <sgopinathan@nvidia.com>
Signed-off-by: Chirag Shah <chirag@nvidia.com>